Quiet please -- Britain's Queen Elizabeth is preparing to have her swans counted.

Buckingham Palace has announced that the annual Swan Upping, a tradition dating back to the 12th century which involves a census of the swan population on the River Thames, will be conducted by the queen's official Swan Marker from July 20-24.

"With the assistance of the Queen's Swan Warden, Professor Christopher Perrins of the University of Oxford, the swans and young cygnets are also assessed for any signs of injury or disease," Buckingham Palace said in announcing the count.

The process involves the Swan Marker, David Barber, rowing up the Thames for five days with the Swan Warden in traditional skiffs while wearing special scarlet uniforms and counting, weighing and measuring swans and cygnets.

It may seem eccentric, but it is very important to the queen.

According to custom, Britain's sovereign owns all unmarked, mute swans in open water, but the queen now exercises the right only on stretches of the Thames and its nearby tributaries.

In medieval times, the Swan Marker would not only travel up the river counting the swans, but would catch as many as possible as they were sought-after for banquets and feasts.

This year, the Swan Marker and the Swan Warden are particularly keen to discover how much damage is being caused to swans and cygnets by attacks from dogs and from discarded fishing tackle.

It is also an important year because Queen Elizabeth has decided to join her team of Swan Uppers for part of the census.

She will follow them up the river and visit a local school project on the whole subject of swans, cygnets and the Thames.

"Education and conservation are essential to the role of Swan Upping and the involvement of school children is always a rewarding experience," Buckingham Palace said.